This application is based on Japanese Patent Application No. 11-150448 filed May 28, 1999, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to a swash plate type compressor, and more particularly to the configuration of a single-headed piston of such type of compressor.
2. Discussion of the Related Art
There has been used a swash plate type compressor equipped with a plurality of single-headed pistons. The compressor of this swash plate type includes (1) a cylinder block having a plurality of cylinder bores formed therein such that the cylinder bores are, arranged along a circle, (2) a rotary drive shaft having an axis of rotation aligned with a centerline of the above-indicated circle, (3) a swash plate rotated with the rotary drive shaft, and (4) a plurality of single-headed pistons each of which includes a head portion slidably engaging a corresponding one of said cylinder bores, and a neck portion engaging said swash plate. In this compressor, the pistons are reciprocated by the swash plate rotated with the rotary drive shaft. An example of this swash plate type compressor is disclosed in JP-A-9-203378. In the swash plate type compressor disclosed in this publication, the head portion of each piston is formed with a through-hole substantially parallel to the circumferential direction of the cylinder block, so as to reduce the mass of the piston. As described below, the head portion of the piston has a relatively high sliding surface pressure at two circumferential portions of its outer circumferential surface which correspond to respective circumferential portions of the cylinder bore at respective radially outermost and innermost portions of the cylinder block, and a relatively low sliding surface pressure at circumferential portions of its outer circumferential surface which are intermediate between the above-indicated two circumferential portions in the circumferential direction of the cylinder block. This fact permits the above-indicated through-hole to be formed through the head portion, for the purpose of reducing the mass of the head portion.
However, the swash plate type compressor described above suffers from a problem that the head portion is subject to a local wear and has an insufficient degree of durability due to a tendency of inclination of the pistons within the cylinder bores. Where the outer circumferential surface of the head portion of each piston is coated with a coating film such as a film of polytetrafluoroethylene (PTFE), this coating film is subject to a local wear, and is relatively likely to suffer from a peel-off problem. Reference is made to FIG. 10, wherein a resultant force Fo consisting of an inertial force of a piston 200 and a force based on a pressure of a refrigerant gas in the cylinder bore acts on a swash plate 202 through a hemispherical shoe 201 (one of a pair of hemispherical shoes). The resultant force Fo is balanced with an axial component Foxe2x80x2 of a force Fa which acts on the surface of the swash plate 202 in a direction perpendicular to that surface. The axial component Foxe2x80x2 acts on the piston 200 in a direction parallel to the centerline of the piston 200. A radial component Fb of the force Fa which component Fb acts on the piston 200 in the radial direction of the swash plate 202 is called a side force acting in a direction perpendicular to the centerline of the piston 200. This radial component Fb (more precisely, a resultant force consisting of the radial component Fb and a friction force between the swash plate 202 and the hemispherical shoe 201) is balanced with reaction forces Fc, Fd which the piston 200 receives from the inner circumferential surface of a cylinder bore 204. Since the resultant force Fo increases as the piston 200 is moved to its upper dead point in its compression stroke, the reaction forces Fc, Fd are the largest at a point near the upper dead point. In particular, the reaction force Fc is comparatively large near the upper dead point of the piston 200. Accordingly, the coating film such as the PTFE film formed on the outer circumferential surface of the piston 200 is likely to be locally worn or removed.
The present invention was made in the light of the background prior art described above. It is therefore an object of the present invention to provide a swash plate type compressor which has an improved degree of durability while reducing the mass of the pistons. This object may be achieved according to any one of the following modes of the present invention, each of which is numbered like the appended claims and depends from the other mode or modes, where appropriate, to indicate and clarify possible combinations of technical features of the present invention, for easier understanding of the invention. It is to be understood that the present invention is not limited to the technical features and their combinations described below, and that any technical feature described below in combination with other technical features may be a subject matter of the present invention, independently of those other technical features.
(1) A swash plate type compressor including:
a cylinder block having a plurality of cylinder bores formed therein such that the cylinder bores are arranged along a circle;
a rotary drive shaft having an axis of rotation aligned with a centerline of the circle;
a swash plate rotated with the rotary drive shaft; and
a plurality of single-headed pistons each including a head portion slidably engaging a corresponding one of the cylinder bores, and a neck portion engaging the swash plate, each single-headed piston being reciprocated by the swash plate rotated by the rotary drive shaft, the head portion of each single-headed piston including a body portion having a circular shape in transverse cross section, and an outer sliding portion and an inner sliding portion which are disposed between the body portion and the neck portion, the outer and inner sliding portions slidably engaging respective circumferential portions of an inner circumferential surface of the corresponding cylinder bore which correspond to respective radially outer and inner portions of the cylinder block,
and wherein a distance from an end face of the body portion remote from the neck portion to an end of the inner sliding portion on the side of the neck portion is larger than a distance from the end face to an end of the outer sliding portion on the side of the neck portion, and the inner sliding portion has a distal sliding part which is spaced from the end face by a distance of at least 40% of an entire length of the piston in an axial direction of the piston and which has a central angle of not larger than 120xc2x0, the distal sliding part having a length which is at least 5% of the entire length of said piston.
In the swash plate type compressor constructed according to the above mode (1) of the invention, the head portion of each piston includes the body portion, the inner sliding portion and the outer sliding portion. The inner sliding portion has the distal sliding part which is spaced from the end face of the body portion by a distance of at least 40% of the entire length of the piston and which has the central angle of not larger than 120xc2x0 and the length which is at least 5% of the entire length of the piston. This arrangement assures a relatively large distance between the axial positions at which the above-indicated two reaction forces act on the piston. Accordingly, the reaction force Fc corresponding to the given side force Fb can be reduced. Further, the amount of increase of the mass of the piston due to the provision of the inner sliding portion is reduced since the distal length of the distal sliding part and its circumferential dimension (central angle) are minimized to such an extent that assures a surface area of the distal sliding part sufficient to limit its sliding surface pressure to a value not higher than a predetermined upper limit. Accordingly, the durability of the distal sliding part (i.e., the durability of the piston) can be effectively increased while reducing the mass of the piston. Where the distal sliding part is coated with a coating film such as a film of PTFE, the local wear and removal of the coating film can be minimized.
The inner sliding portion may be formed so as to extend from the body portion in the axial direction (such that the outer circumferential surface of the inner sliding portion is continuous with that of the body portion). Alternatively, the inner sliding portion may be formed in spaced-apart relation with the body portion (such that the outer circumferential surface of the inner sliding portion is not continuous with that of the body portion). In the former case, the length or distance from the end face of the body portion to the end of the inner sliding portion on the side of the neck portion is equal to a sum of the axial length of the body portion and the axial length of the inner sliding portion (total axial length of the body portion and the inner sliding portion as measured on the radially inner side of the head portion). In the latter case, the above-indicated distance is larger than the above-indicated sum. The above description applies to the outer sliding portion. Namely, the outer sliding portion may either extend from the body portion, or be formed in spaced-apart relation with the body portion. Further, the relationship described above with respect to the inner sliding portion applies to the relationship between the distance from the end face of the body portion to the end of the outer sliding portion on the side of the neck portion and the sum of the axial lengths of the body portion and the outer sliding portion
The distance from the end face of the body portion to the end of the inner sliding portion on the side of the neck portion is made larger than the corresponding distance of the outer sliding portion, in view of a fact that the durability of the piston can be effectively improved by increasing the above-indicated distance of the inner sliding portion rather than the corresponding distance of the outer sliding portion. In this respect, it is noted that the sliding surface pressure at the end portion of the outer sliding portion on the side of the neck portion is maximized at a point of transition from the suction stroke to the compression stroke of the piston, and that the side force at this point of time is based primarily on the inertial force of the piston, and is smaller than the side force in the terminal portion of the compression stroke. Accordingly, it is more effective to increase the above-indicated distance of the inner sliding portion rather than the corresponding distance of the outer sliding surface.
The circumferential dimension (central angle) of the inner sliding portion may be constant over its entire axial length, or may be smaller or larger at its part nearer to the neck portion than at its part near to the body portion. In the latter case, the central angle of the distal sliding part of the inner sliding portion is made smaller or larger than that of the other part (referred to as xe2x80x9cproximal sliding partxe2x80x9d since it is adjacent or nearer to the body portion of the head portion) of the inner sliding portion. The distal sliding part may be configured such that its central angle is constant over its entire axial length or changes depending upon the axial position. For instance, the central angle of the distal sliding part may decrease continuously or in steps as it extends in the axial direction toward the neck portion. The distal sliding part may be formed integrally with the proximal sliding part, or in spaced-part relation with the proximal sliding part.
The proximal and distal sliding parts may have a suitable shape in transverse cross section, which may be generally defined by an arc and a chord, or by a part of an annulus, or may be crescent or generally rectangular. In other words, the surfaces of the proximal and distal sliding parts of the inner sliding portion which slidably engage the inner circumferential surface of the cylinder bore are required to have shapes which follow the corresponding parts of that inner circumferential surface. However, the surfaces of the proximal and distal sliding parts which are opposed to the outer sliding portion may have any shapes, for instance, may be flat surfaces or concave surfaces. Where these surfaces are concave, the mass of the piston is reduced owing to the concavity. The shapes in transverse cross section may be symmetrical or asymmetrical with respect to a plane which passes the centerline of the piston and the centerline of the cylinder block. As described before, a force of friction between the piston and the swash plate also acts on the piston, so that the direction in which a reaction force produced by the inner circumferential surface of the cylinder bore acts on the piston deviates from the plane passing the centerline of the piston and cylinder block, in a direction determined by the direction of rotation of the swash plate. Accordingly, where the swash plate (drive shaft) is rotated in a predetermined one direction, it is advantageous that the inner sliding portion has an asymmetric shape in transverse cross section such that the inner sliding portion has a larger sliding surface on one side of the above-indicated plane on which the above-indicated reaction force deviates from that plane, than on the other side.
The distal sliding part is spaced from the end face of the body portion by a distance which is at least 40% of the entire length of the piston. Preferably, this distance is at least 43% or 46% of the entire length of the piston. An effect of the distal sliding part to prevent the inclination of the piston relative to the centerline of the cylinder bore increases with an increase in the distance of the distal sliding part from the end face of the body. Where the distal sliding part is formed integrally with the proximal sliding part, the weight of the piston increases with the above-indicated distance. It is further noted that the maximum operating stroke of the piston is determined by the outside diameter and inclination angle of the swash plate. Therefore, the axial position of the distal sliding part is desirably determined by taking into account its effect to prevent the piston inclination, the amount of increase of the piston mass and the operating stroke.
(2) A swash plate type compressor according to the above mode (1) , wherein the central angle of the distal sliding part is not larger than 100xc2x0.
While the central angle of the distal sliding part is required to be not larger than 120xc2x0 according to the above mode (1), this central angle is preferably not larger than 110xc2x0 or 100xc2x0, for effectively reduce the mass of the piston. The mass of the piston can be more effectively reduced when the central angle is 95xc2x0 or 90xc2x0 or smaller.
A decrease in the central angle of the distal sliding part increases the sliding surf ace pressure of the distal sling part, but reduces the mass of the piston. Accordingly, the central angle of the distal sliding part is preferably determined by taking into account both the sliding surface pressure and the piston mass. Where the central angle of the distal sliding part is extremely small, for example 20xc2x0, the inner sliding portion having this distal sliding part provides some effect to prevent inclination of the piston relative to the centerline of the cylinder bore, as compared with the inner sliding portion which does not have the distal sliding part. In view of this, the central angle of the distal sliding part may be not larger than 85xc2x0, 80xc2x0 or 70xc2x0.
(3) A swash plate type compressor according to the above mode (1) or (2), wherein the inner sliding portion includes a wide section disposed. on the side of the body portion, and a narrow section which is disposed on the side of the neck portion and which has a smaller circumferential dimension than the wide section.
The narrow section and the wide section may be respectively the distal sliding part and the proximal sliding part which have been described above.
(4) A swash plate type compressor according to the above mode (1) or (2), wherein the inner sliding portion includes a narrow section disposed on the side of the body portion, and a wide section which is disposed on the side of the neck portion and which has a larger circumferential dimension than the narrow section.
The narrow section and the wide section may be respectively the proximal sliding part and the distal sliding part which have been described above. This arrangement permits effective reduction of the sliding surface pressure while reducing the increase of the mass of the piston due to the provision of the inner sliding portion.
(5) A swash plate type compressor according to any one of the above modes (1)-(4), wherein the length of the distal sliding part is at least 8% of the entire length of the piston.
The axial length of the :distal sliding part is at least 5% of the entire length of the piston according to the principle of the above mode (1) of the present invention. In the above mode (5) wherein this axial length is at least 8% of the entire piston length, the sliding surface pressure of the piston can be further reduced. Where the length of the distal sliding part is at least 10%, 12% or 15% of the entire piston length, the piston can be further effectively prevented from being inclined.
Where the inner sliding portion extends continuously from the body portion in the axial direction, an increase in the axial length of the distal sliding part increases the entire axial length of the inner sliding portion and consequently the mass of the piston, if the axial length of the proximal sliding part is unchanged. Accordingly, the percentage of the axial length of the distal sliding part with respect to the entire piston length is desirably determined by taking account of the effect to reduce the sliding surface pressure of the piston, and the amount of increase in the mass of the piston due to the provision of the inner sliding portion.
(6) A swash plate type compressor including:
a cylinder block having a plurality of cylinder bores formed therein such that the cylinder bores are arranged along a circle;
a rotary drive shaft having an axis of rotation aligned with a centerline of the circle;
a swash plate rotated with the rotary drive shaft; and
a plurality of single-headed pistons each including a head portion slidably engaging a corresponding one of the cylinder bores, and a neck portion engaging the swash plate, each single-headed piston being reciprocated by the swash plate rotated by the rotary drive shaft,
the head portion of each single-headed piston including a body portion having a circular shape in transverse cross section, and an outer protrusion and an inner protrusion which extend toward the neck portion from respective radially outer and inner portions of the cylinder block and which slidably engage respective circumferential portions of an inner circumferential surface of the corresponding cylinder bore,
and wherein a total length of the body portion and the inner protrusion in an axial direction of the piston is at least 45% of an entire length of the piston, the inner extension having a central angle of not larger than 120xc2x0 in at least a distal end portion thereof which is remote from the body portion and whose axial length is at least 10% of the above-indicated total length, a distance of extension of the inner protrusion from the body portion being larger than that of the outer protrusion.
In the swash plate compressor constructed according to the above mode (6) of the present invention, the outer sliding portion and the inner sliding portion of the head portion are provided in the form of the outer protrusion and the inner protrusion, respectively which extend from the body portion of the head portion in the axial direction toward the neck portion. The head portion including these outer and inner protrusions has a larger strength than the head portion wherein the outer and inner sliding portions are formed in spaced-apart relation with the body portion.
The distal end portion of the inner protrusion whose central angle is not larger than 120xc2x0 and whose axial length is at least 10% of the total length of the body portion and the inner protrusion corresponds to the distal sliding part described above with respect to the above mode (1). The distal sliding part may be referred to as a sliding distal end part.
(7) A swash plate type compressor including a cylinder block having a plurality of cylinder bores formed therein such that the cylinder bores are arranged along a circle, a rotary drive shaft having an axis of rotation aligned with a centerline of the circle, a swash plate rotated with the rotary drive shaft, and a plurality of single-headed pistons each including a head portion slidably engaging a corresponding one of the cylinder bores, and a neck portion engaging the swash plate, each single-headed piston being reciprocated by the swash plate rotated by the rotary drive shaft,
the head portion of each single-headed piston including a body portion having a circular shape in transverse cross section, and an inner sliding portion including (a) an inner protrusion which extends toward the neck portion from a radially inner portion of the cylinder block and which has a proximal inner sliding surface which slidably engages an inner circumferential surface of the corresponding cylinder bore, and (b) a spaced-apart distal sliding part which has a spaced-apart inner sliding surface spaced apart from the inner protrusion, the spaced-apart distal sliding part being spaced from an end face of the body portion remote from the neck portion, by a distance of at least 40% of an entire length of the piston, the spaced-apart distal sliding part having a central angle of not larger than 120xc2x0 and an axial length which is at least 5% of the entire length of the piston.
In the swash plate type compressor constructed according to the above mode (7), the distal sliding part is the spaced-apart distal sliding part which is spaced from the inner protrusion. For instance, the spaced-apart distal sliding part may be formed on a connecting portion which connects the inner protrusion and the neck portion. The spaced-apart distal sliding part is located at an axial position between the inner protrusion and the neck portion, which axial position is spaced from the end face of the body portion by a distance of at least 40% of the entire piston length, as described above. This mode (7) of the invention provides an increased freedom of design in the position of the spaced-apart distal sliding part.
The above mode (7) may be modified such that the inner protrusion is a connecting portion which does not have the proximal inner sliding surface and which merely connects the spaced-apart distal sliding part and the body portion.
(8) A piston for a swash plate type compressor, said piston having a head portion slidably received in a cylinder bore formed in a cylinder block, and a neck portion engaging a swash plate, the head portion includes:
a body portion having a circular shape in transverse cross section; and
an outer sliding portion and an inner sliding portion which are disposed between the body portion and the neck portion, the outer and inner sliding portions slidably engaging respective circumferential portions of an inner circumferential surface of the corresponding cylinder bore which correspond to respective radially outer and inner portions of the cylinder block,
and wherein a distance from an end face of the body portion remote from the neck portion to an end of the inner sliding portion on the side of the neck portion is larger than a distance from the end face to an end of the outer sliding portion on the side of the neck portion, and the inner sliding portion has a distal sliding part which is spaced from the end face by a distance of at least 40% of an entire length of the piston in an axial direction of the piston and which has a central angle of not larger than 120xc2x0, the distal sliding part having a length which is at least 5% of the entire length of the piston. the each
There is also provided a piston for a swash plate type compressor, which is described with respect to any one of the above modes (2)-(7).